DESCRIPTION : It is the purpose of this grant to examine the basic physiologic mechanisms underlying the response of gait and alertness to nocturnal light exposure. It is our intent to demonstrate that exposure to specific wavelengths of light at night will be able to improve gait (specific aspects of which are useful proxies for predicting likelihood of falling) while simultaneously not affectig alertness and subsequent sleep in older individuals. The experiment will take place in a specially-designed sleep/circadian laboratory at the VA Palo Alto Health Care System. After a screening visit and randomization, healthy, older individuals (n=27) will spend a week at home during which time they will keep a regular sleep-wake cycle to ensure good sleep prior to entry into the laboratory. On the evening of the laboratory overnight stay, subjects will have a baseline assessment of gait, alertness and visual acuity. Subjects will go to sleep at their habitual bed time and be awoken two hours later, at which time they will be kept awake for the next 13 minutes. During this 13-minute block, subjects will be exposed to one of three experimental lighting regimes (n=9 in each condition): very dim white light (approximating not turning on overhead lighting at night when getting up to use the toilet), white room light (approximating turning on standard overhead lighting), or dim orange light (>550 nm). The colored light condition is hypothesized to be the equipoise between minimizing the effects of light on alertness, which has a peak sensitivity to blue light, and maximizing the effects of ligh on visual acuity and gait. During this 13- minute exposure to light, subjects will be tested for gait, alertness, and visual acuity. After this, subjects will be allowed to return to sleep and the discharged in the morning. Given previous research, we hypothesize that in dim white light, there will be little change in alertness, gait, and visual acuity over the 13-minute testing period We hypothesize that in the white room light, there will be an increase in alertness and improvement in both gait and visual acuity. We hypothesize that in the dim orange light, there will be no change in alertness and an improvement in both gait and visual acuity. The dim orange light is to be composed of wavelengths of visible light that are predicted to have a significant impact on image formation (i.e., conscious visual perception), but have minimal impact on the alerting effects of light on the brain. These latter effects are mediated in part by network of intrinsically photosensitive retinal ganglion cells and have maximal sensitivity to blue light. In addition to achieving a greater understanding of the physiology underlying the effects o light on balance, gait, and alertness, this research could lead to direct application - the design of a specialized night light that would enable older individuals to use the toilet at night withut waking themselves up so much that it becomes difficult to fall back to sleep. This specialized night light would enable people to have enhanced image formation at night, which will increase their confidence in walking to the toilet and reduce likelihood of falling while doing so, while simultaneously not increasing their alertness, thereby making it easier to return to sleep. Given the age of the Veteran demographic and the frequency of using the toilet at night in older adults who are at risk for both falling and insomnia, this could be of significant value to Veterans and the VA. PUBLIC HEALTH RELEVANCE: We have a rapidly aging Veteran population. Even in those older individuals who are healthy, falling is a significant health risk. This risk is further increased at night and exacerbated by th types of vision loss common in older adults. The frequency of night time ambulation is elevated in many older adults due to an increased rate of nocturnal enuresis. In getting up to use the toilet at night, individuals will commonly turn lights on to aid in balance and navigation so as to reduce some of the risk of falling. One unintended consequence of turning on the lights during the night is that this light directly activates the brain and makes it significantly more difficultto return to sleep. Different light-responsive systems in the eye underlie the conscious visual (image formation) and alerting effects of light. A better understanding of this physiology would allow us to design a specialized night light that would improve nocturnal navigation and balance without disrupting sleep.